1. Field of the Invention
The present invention generally relates to optical systems for automated imaging and, more particularly, to optical systems for digital imaging for optical character recognition (OCR), automated inspection, optical metrology (e.g. object identification and classification), machine vision, robotics, bar code readers (BCRs) and the like.
2. Description of the Prior Art
Many different types of optical systems including one or more lenses and/or mirrors and structure to maintain or adjust the relative positions of such optical elements have been known for many years for various applications, including telescopes, microscopes and human vision correction. Cameras generally combine such optical systems with an arrangement to position a radiation sensitive film or electronic sensor positioned relative to the focal plane of the optical system. Cameras using electronic sensors have recently become very popular as a substitute for film-based cameras and have also found substantial utility in various automated image capture applications such as vision-based control systems, BCRs and OCR systems.
Many different types of optical systems are known and have different properties which may be of potential relative advantage in particular applications. However, application of some types of optical systems to purposes in which they might be otherwise advantageous may be precluded by corresponding constraints. As a simple example, large fields of view must generally be provided by wide-angle lenses which characteristically exhibit substantial distortion that increases with off-axis angle. Correction of such distortion is often only possible with expensive aspheric elements or through the use of image processing which is expensive.
Long focal length lenses at considerable distances from an object to be imaged have been used to restrict the relative change in resolution over a large change in object position along the optical axis but this can be impractical where space is a limitation and may require the use of multiple path-folding mirrors of high optical quality which are expensive and ultimately reduce overall image quality because of surface irregularities. This is, at best, only a partial solution since the resolution will still be relatively high at relatively closer object positions thus limiting unnecessarily the ultimate scan rate of the image capture device. Therefore, some types of lens systems exhibiting some potentially useful properties have little or no known practical application and are often, at most, of theoretical interest.
So-called telecentric optical systems are exemplary of systems having unique properties but which are severely constrained in application. An article entitled “Optical Design and Specification of Telecentric Optical Systems” by Michael A. Pate, Proc. SPIE, Vol 3482, pp. 877–886, Jun. 8–12, 1998, which is hereby fully incorporated by reference, notes that the objective lens must be larger than the field of view or object of interest and that telecentric optical systems have principal utility in measurement and fabrication of three-dimensional parts. The article also notes that while telecentric lenses were independently discovered in 1848 (Porro) and 1878 (Abbe), telecentric optical system design is not well known or published; stating that the published literature is “very elementary” and “current optical design literature uses anywhere from one sentence to one paragraph to describe and define telecentric optical system design”.
By definition, telecentric optical systems fall into two main categories, image telecentric systems and object telecentric systems. Image telecentric systems have an aperture located at the front focal plane of the objective lens such that the chief ray from any object point passes through the center of the aperture and emerges parallel to the optical axis in image space and perpendicular to the image plane (exit pupil at infinity). Object telecentric systems have an auxiliary lens (or mirror) which is located such that its back focal plane coincides with the entrance pupil of the objective lens such that the chief ray from any object point is parallel to the optical axis in object space (entrance pupil at infinity). These two categories can be combined to form a hybrid system known as a doubly telecentric system. This geometry provides the theoretical property of providing constant size imaging over a range of distances of an object from the optical system. Thus the image of an object formed by a doubly telecentric system will be substantially isometric or orthographic (whereas an image made with a non-telecentric system will have substantially a single vanishing point perspective).
However, telecentric optical systems, in practice, fall into two groups and a combination or hybrid group, each having a combination of severe constraints. Object telecentric optical systems, which require an auxiliary objective, are telecentric on the object side of the objective only and utilize the aperture of the camera objective lens. Practical use of object telecentric optical systems relies on depth of field (implying a small aperture and limited light-gathering ability) and constant focus. As long as focus does not change, the center of blur (circle of confusion) of an imaged point from an object will remain stationary in the image plane as the object being imaged is moved along the lens system axis.
That is, within a relatively shallow depth of field and for a field smaller than the auxiliary lens diameter, the image of an object will be of constant size over a range of axial motion of the object. However, the property of a constant size image is lost if the system is refocused in accordance with such axial motion. Therefore, an extremely restrictive range of axial motion (within the depth of field at a given object distance) and object size (limited by the diameter of the auxiliary lens) must be observed in order to render focused detail of the object at constant image size.
As an example of such limitations, U.S. Pat. No. 4,851,698 to Hippenmeyer discloses use of an object telecentric system for measuring separation of features on an object surface. In this case, a well-focused image is apparently of relatively low importance (in connection with a row camera having a linear array of sensors) so long as the image blur allows determination of the respective centers of blurred image features. The lens diameter must also exceed the distance to be measured on the object but can be extended by a factor of ten along a linear zone corresponding to a row of detectors by use of a concave strip mirror. Further, the system must be recalibrated if it is refocused to closer distances than infinity (which will also reduce usable depth of field, referred to therein as a range of sharpness).
So-called image telecentric optical systems are somewhat the reverse of object telecentric systems. Image telecentric systems have an aperture at the front focal plane. However, unlike object telecentric lenses, alteration of focus does not alter the magnification and size of the image provided that the object and the lens elements remain stationary relative to one another, i.e., if only the image sensor is moved to adjust focus. However, motion of the object along the optical axis results in a change of magnification of the image, focused or unfocused. In essence, an image telecentric lens system has virtually no advantages over an object telecentric lens system and significant further constraints.
Even though known measurement applications use telecentric lens systems at a single focus distance, this may be a critical constraint in regard to some possible applications and further limits use of object and image telecentric lenses in applications where a focused image is of importance. The hybrid configuration alluded to above is referred to as a doubly telecentric optical system and essentially uses both an image telecentric objective coaxially located with an object telecentric objective such that the rear focal plane of the object telecentric objective coincides with the front focal plane of the image telecentric objective with the aperture placed at the juncture of the two focal planes serving as the exit aperture of the object-side objective and the entrance pupil of the image-side objective.
However, it has generally been considered that a doubly telecentric system would be constrained by the constraints characteristic of both object and image telecentric optical systems, especially by the constraint in the field width due to limited width of the object-side objective which is traditionally a refractive lens. Accordingly, little, if any, practical application has been envisioned for doubly telecentric optical systems. The properties of doubly telecentric optical systems do not appear to have been investigated and appear to be considerably less well-known than object or image telecentric systems.
In fact, the above-incorporated article, while otherwise seeking to be a comprehensive overview of telecentric lens design, does not even acknowledge the existence of doubly telecentric optical systems, nor does it include any discussion of the properties or applications of such a configuration. It is also significant that while object telecentric and image telecentric refractive lenses are commercially available, at least in small diameters, doubly telecentric lenses are not similarly available and cannot reasonably be assembled from commercially available object and image telecentric systems.
The above-incorporated article also acknowledges that the large lens sizes required by telecentric optical systems can be extremely expensive since the cost of a lens generally increases as the cube of the diameter. Therefore, while the article acknowledges that image processing by computer is complex and slow, particularly in correcting for distortions of non-telecentric lenses used for making optical measurements, such image processing has been more economical than use of large telecentric optical systems which do not change magnification with lens to object distance, at least for measurement applications.
Nevertheless, digital imaging quantizes the image at least in accordance with scan lines or the array of sensor areas at which image capture can occur. With known and well-understood non-telecentric optical systems, magnification and, hence, resolution vary with changes in distance of an object from the camera lens or focal plane. Even with object telecentric lens systems, resolution will necessarily vary with distance of the object from the lens in the sense that the image will become defocused with change of distance between the object and lens outside a shallow depth of field if the optical system is operated at a fixed focus or, if the lens is refocused, magnification and, hence, resolution will change.
This characteristic of most lenses presents a problem in regard to optical character recognition (OCR), bar code readers (BCRs), feature extraction systems and the like which must also operate from data representing a reasonably well-focused image (e.g. systems which compare image features with templates). This comparison, especially in systems including some degree of adaptive processing, requires constant resolution from one image to the next.
Similarly, optical metrology, requires constant image magnification even where the distance between the object of interest and the optical system may not be well-controlled. Variations from these requirements may also affect the performance of other systems such as bar-code readers and optical inspection systems to varying degrees. Magnification variation with depth/distance also causes severe perspective distortion (particularly when providing wide angles of view) which present severe difficulties for machine vision systems such as vision based guidance systems and robotics. Generally, a relatively wide angle of view is required to accommodate the size of objects of interest while maintaining the overall system (object, lens and camera/sensor) at an acceptable size even though, as alluded to above, distortion is usually increased with wide-angle lenses.
It should also be appreciated, in this regard, that there are some environments where the distance between the lens and features of interest on an object inherently varies widely and cannot be controlled. For example, when using OCR systems to read address information on packages of essentially random sizes, the lens to object distance will necessarily vary as widely as the maximum size to be accommodated since some packages could be very thin or short. In the case of tall packages which would place the address information very close to the lens, the dimensions of the information may exceed the field of view of the optical system.
Further, as OCR and optical metrology techniques are applied to high throughput systems such as mail sorting, the digital image processing overhead to correct for change of magnification with distance and distortion becomes a major limitation on overall system capacity, even when very high speed digital data processors are employed. Such image processing necessarily carries a substantial computational overhead to achieve the effect of essentially discarding data and the resolution that data represents and which could otherwise be utilized to improve machine vision system performance. Additionally, this limitation on throughput carries a significant economic cost in regard to the controlled system which must increasingly be balanced against optical system cost when high throughput is required.
Moreover, OCR, BCR and other machine vision systems must capture a reasonably well-focused image for further processing. It was noted above that object telecentric systems cannot be focused without recalibration of image size and must be used to image objects in a relatively narrow depth of field where focus errors are of a tolerable magnitude. Likewise, image telecentric systems can only be focused at a constant image size by movement of the image plane whereas traditional focusing techniques move either the entire lens or subgroups of elements necessitating movement of the object-side objective in synchrony with the image-side objective. However, any change in object position relative to the objective changes image magnification. This difficulty of focusing even image telecentric systems without changing the magnification is thus seen to be a severe limitation on applicability of telecentric optical systems to environments where well-focused images are required.